1. Field of the Invention
The present invention relates to a constitution in the case where lamp annealing is utilized as a heating treatment method frequently used in a fabrication process of a semiconductor device. Particularly, the present invention is effective in fabricating a semiconductor device such as a thin film transistor (TFT) on a glass substrate.
2. Description of Related Art
In recent years, development of TFT formed by utilizing a semiconductor thin film (typically thin film whose major component is silicon) on a glass substrate has significantly been progressed. Further, demand of an electrooptical device in which a pixel matrix circuit, a driver circuit, a logic circuit and the like are monolithically mounted on a glass substrate has been promoted.
The most significant restriction caused in forming TFT on a glass substrate is temperature of process. That is, a restriction whereby a heating treatment cannot be carried out at a heat resistant temperature of glass or higher narrows the margin of process.
Therefore, laser annealing process has been utilized as a means for annealing selectively a thin film. According to the laser annealing process, only a thin film can selectively be heated by elevating instantaneously temperature of a sample by irradiating a pulse laser beam onto the sample. However, there has been posed a problem in view of mass production steps in which an optical system is complicated to deal with a laser beam and the uniformity is difficult to ensure.
Hence, a lamp annealing process using a strong beam emitted from an arc lamp, a halogen lamp or the like has recently been spotlighted. This technology is referred to as RTA (Rapid Thermal Annealing) or RTP (Rapid Thermal Processing) in which a film to be treated is heated by irradiating a strong beam in a region of wavelength that is apt to be absorbed by the film to be treated.
Normally, the lamp annealing process utilizes a region of visible light to infrared light as strong beam. The light in this wavelength region is difficult to absorb by a glass substrate and accordingly, the heating of the glass substrate can be restrained to a minimum. Further, time periods for temperature rise and temperature drop are extremely short and accordingly, high temperature treatment at 1000° C. or higher can be carried out in a short period of time of several seconds to several tens seconds.
Further, a complicated optical system such as used in a fabrication process by using a laser beam is not needed and therefore, the process is suitable for treating a comparatively large area with excellent uniformity. Also, the yield and throughput are promoted since the high temperature treatment is basically carried out by a sheet by sheet process.
It is a problem of the present invention to improve the above-described lamp annealing process and to provide a method for effectively subjecting a film to be treated to a heating treatment.
Further, according to the conventional lamp annealing process, light has been irradiated only from an upper face side of a film to be treated and therefore, when a layer which does not transmit the light (for example, electrode made of a metal) or a layer which hinders irradiation of light is present at a portion or a total face of the film to be treated, the film to be treated beneath the layer could not be annealed.
Particularly, when the conventional lamp annealing process was used in a step of activating impurities doped in a semiconductor thin film, an electrode made of a metal and an insulating film which were laminated on the semiconductor thin film hindered irradiation of light and source/drain regions excellent in uniformity could not be formed.
It is one of the problems of the present invention to provide a semiconductor thin film having source/drain regions excellent in uniformity by activating impurities through a step using a heating treatment method improving the conventional lamp annealing process in a semiconductor thin film doped with impurities and by heat treatment at later steps.